This invention relates to gaging standards for the ultra-precise measurement of length and is more particularly directed to a method and apparatus for increasing the accuracy with which a laser interferometer will measure the distance between the end faces of length standards.
0.05 LASER INTERFEROMETERS ARE CAPABLE OF MEASURING TO ACCURACIES OF LESS THAN SEVERAL MILLIONTHS OF AN INCH (.05 MICRONS), CONSIDERABLE DIFFICULTY HAS BEEN ENCOUNTERED IN ATTAINING SUCH HIGH DEGREE OF ACCURACY IN THE MEASUREMENT OF THE LENGTH OF GAGING STANDARDS IN THE FORM OF BARS OR RODS. Such standards are customarily 20 inches (50.8 cm.) or more in length and are therefore ordinarily supported in a horizontal plane in order to avoid the foreshortening which would otherwise occur due to the increase in weight along the vertical axis when supported on one end face thereof. In current gaging set-ups for use with laser interferometers, the length standard to be measured it usually supported between centers, or is suitably clamped on a gaging table, while the gaging head is arranged to ride on one or more rails located in offset but parallel relation to one side of the standard. The sensing probe generally extends outwardly from the gaging head at right angles to the length axis of the standard while the target mirror from which the laser beam is reflected is conventionally attached to the gaging head on the side opposite the extending probe. While the end faces of length standards are parallel to each other within several millionths of an inch (0.05 microns), these faces are not necessarily in squared relation with the connecting sides therebetween to the same degree of accuracy. Consequently, the axis of travel of the probe as well as that of the target reflector may not be in true parallel alignment with the axis of the distance to be measured. Such condition is known in the field of metrology as "cosine error" and will, of course, detract from the desired accuracy of measurement.
Another undesirable characteristic of conventional fixturing equipment for laser interferometer installations lies in the offset relationship between the axis of the measured distance and the axis of the laser beam. Thus, in the event the rail surfaces are not perfectly straight, or are otherwise not fully aligned with the sides of the length standard, the degree of such misalignment will be magnified in direct ratio to the offset distance betwen the axis of the length standard and the center of the target from which the laser beam is reflected. Such condition, referred to in the art as "Abbe offset error" also detracts from the accuracy of the interferometer measurements.
Accordingly, it is an object of this invention to provide a method for the ultraprecise measurement of the distance between the end faces of a horizontally supported length standard even though the end faces thereof are not perfectly square with the connecting sides therebetween.
Another object of this invention is to provide a method of measuring, as aforesaid, wherein the distance between the end faces of a length standard is determined by a laser interferometer arranged so that the axis of the laser beam coincides with the axis of measurement of the length standard.
Still another object of this invention lies in the provision of a laser interferometer installation, as aforesaid, wherein the target mirror from which the laser beam is reflected and the sensing probe which determines the location of the target mirror relative to a known reference point are both mounted on the same positioning fixture in respective coaxial alignment.
A further object of this invention is to provide an interferometer installation, as aforesaid, wherein the accuracy of measurement is not dependent on the precision of such geometrical relationships of the sides of the positioning fixture as straightness, squareness, and parallelism.
An additional object of this invention is to provide an interferometer installation, as aforesaid, wherein the contact between the gaging probe and each of the surfaces defining the limits of the distance to be measured is automatically controlled to provide identical gaging pressures therebetween.
Another object of this invention is to provide control means, as aforesaid, which compensates for variations in the force with which the gaging probe is brought into contact with the fixed surfaces defining the distance to be measured.
A still further object of this invention lies in the provision of adjustable swivel means for orienting the fixed surface in abutment with one end face of the length standard to coincide with such end face to within one millionth of an inch (0.025 microns).